Afferent feedback plays a critical role in the control of locomotion and in the recovery afforded by bodyweight supported treadmill training or grafts of neurotrophin producing cellular transplants after spinal cord injury (SCI). Yet our knowledge of the structure of the locomotor circuitry is limited and the roles of afferent feedback in locomotor recovery have been for the most part unexplored. This multidisciplinary project will integrate a neural model of the locomotor circuitry developed at Drexel University with a biomechanical hindlimb model developed at Georgia Institute of Technology. This integrated model will be used as a test bed to study the role of afferent feedback in the control of locomotion in the intact cat and in the recovery of locomotor function after spinal transection. We hypothesize (and have demonstrated using a simplified model) that muscle length dependent feedback from the hip muscles is critical for the initiation of swing and that force feedback from the ankle extensors and cutaneous input from the footpad are critical for the control of stance. We also hypothesize that in spinal cats, training or neurotrophin producing transplants enhance the synaptic strengths of the sensory feedback and that these increases enable the circuitry to produce a stable locomotor pattern in the absence of supraspinal control. The model will allow us to investigate the above key hypotheses and make predictions about the motor pattern deficits obtained when certain sensory modalities are removed after SCI. Experiments with partially deafferented or re-innervated muscles (muscles without length feedback) will be performed to test these predictions and further refine the model. The aims of this project are consistent with the mission of the National Institute of Biomedical Imaging and Bioengineering to develop and accelerate the application of biomedical technologies and integrate the physical and engineering sciences with the life sciences to advance basic research and medical care. The insights obtained through the model and experimental studies about the roles of afferent feedback in locomotor recovery following SCI may guide rehabilitation training efforts in individuals with spinal cord injury.